Graphite is a form of carbon with the carbon atoms bonded in layers with weaker bonds between the layers. This structure allows intercalation by additional atoms or molecules which occupy spaces between the carbon layers. Graphite can be intercalated by exposure to an appropriate chemical reagent, known as the intercalate, which allows the intercalate to enter between the carbon layers of the graphite. Intercalation can be performed, for example, by immersing graphite flakes in red fuming nitric acid for a day or more. The resulting material, known as intercalated graphite or a graphite intercalation compound, comprises carbon layers and intercalate layers stacked on top of one another in a periodic fashion. For example, the stacking can be of the form --C--C--I--C--C--I--C--C--I--C--, where C is a carbon layer, and I is an intercalate layer. The number of carbon layers between the nearest intercalate layers is known as the stage. For example, --C--C--I--C--C--I--C--C--I--C--C--I-- represents a stage 2 intercalated graphite.
Heating intercalated graphite flakes to a sufficiently high temperature causes exfoliation, which is a sudden increase in the dimension perpendicular to the carbon layers of the intercalated graphite, forming vermicular or wormlike shapes. The exfoliated graphite flakes, or worms, are expanded up to hundreds of times along the c-axis of the graphite, which is the axis perpendicular to the carbon layers.
Forming exfoliated graphite from graphite flakes is described in U.S. Pat. Nos. 1,191,383, and 3,404,061, and from pyrolytic graphite in U.S. Pat. No. 3,404,061. Generally these patents describe formation of exfoliated graphite flakes by first intercalating graphite flakes with an intercalating agent, such as nitric acid, and subsequently heating the intercalated flakes to an elevated temperature, typically to about 600.degree. C. or higher.
Composites comprising compressed exfoliated graphite and a polymeric resin have been made. For example, Alysworth in U.S. Pat. Nos. 1,137,373 and 1,191,383 discloses composites made by treating exfoliated graphite with a phenol-formaldehyde binder and compressing the binder-treated graphite into an integral mass.
Shane et al. in U.S. Pat. No. 3,404,061 disclose a process wherein vermiform graphite is compressed without a binder to form a soft, flexible sheet material. Impregnants or additives, such as metal powders, clays, or organic polymer materials may be added.
Olstowski et al. in U.S. Pat. No. 3,492,197 disclose the production of composites of compressed vermicular or exfoliated graphite and a polymeric resin. The composites are produced by compressing mixtures of exfoliated graphite and a polymeric resin at a pressure between 5 and 50,000 psi (34 kPa-340 MPa), sufficient to compress the graphite in the composite.
Vogel et al. in U.S. Pat. No. 4,414,142 disclose a highly conductive composite comprising an organic polymer matrix with an unexfoliated graphite intercalation compound. The composite may be formed by molding together a mixture of intercalated graphite, and a polymeric resin at a molding temperature below about 160.degree. C., which is low enough so as not to affect or destroy the intercalated graphite, or i.e., to cause the transformation of the intercalated graphite into an exfoliated state.
In forming composites of exfoliated graphite and a polymer by prior art methods, the graphite must be intercalated and then exfoliated, after which the exfoliated graphite is mixed with a liquid resin which is then solidified to form a composite of graphite and the resin. It would be desirable to form composites of exfoliated graphite without the necessity of a previous exfoliation step.